1. Field of the Invention
This invention relates to a method for processing lithographic printing material in accordance with a silver salt diffusion transfer process and a developer used therein.
2. Prior Art
Offset printing plates utilizing silver images obtained by the silver complex diffusion transfer reversal (DTR) process are known from, for example, Japanese Patent Publication (JP-B) Nos. 43132/1971 and 30562/1973. Positive printing original plates are described in Japanese Patent Application Kokai (JP-A) No. 55402/1974 and negative ones are described in JP-A 106902/1977, JP-A 112402/1977, and EP 481562 A1.
These direct printing plates generally include an undercoat layer also serving for anti-halation, a silver halide emulsion layer, and a physical development nucleus layer on a support. On treatment of photosensitive material after imagewise exposure with a developer containing a developing agent and a silver halide solvent, silver halide having a latent image formed therein is converted to blackened silver in the emulsion layer while silver halide having no latent image therein is dissolved by the action of a silver halide complexing agent and diffused to the surface of the photosensitive material. The reducing action of the developing agent causes the silver complex salt to precipitate on physical development nuclei in the surface layer. A printing plate is obtained in this way. If desired to enhance the ink receptivity of silver images, development is followed by sensitization. The printing plate is mounted on an offset printing machine whereby ink images are transferred to ink-receptive sheets.
If solution and transfer take place prior to or during development, this lithographic printing plate is reduced in resolving power and fails to produce satisfactory transfer images. In order that development take place faster than solution, several proposals were made including the combination of a photosensitive material having a developing agent incorporated therein with an activator type solution (pH 13.0 or higher) and the use of a developer at pH 12.0 or higher to provide an increased development rate. However, the processing solution which is of the activator type or of higher pH is hazardous in handling, absorbs more carbon dioxide gas from air, experiences a frequent pH variation, and renders the developing agent susceptible to air oxidation.
On the other hand, if the developer for diffusion transfer is lowered in pH in order to improve the stability thereof, the development rate is retarded and the dissolving rate is relatively increased, failing to produce satisfactory transfer images.
For example, JP-A 071056/1984, 141561/1987, 282750/1990, and 049535/1984 disclose developers containing less than 20 grams of hydroquinone. JP-A 179744/1985, 071055/1984, 105147/1987, and 287357/1990 disclose activator solutions free of a developing agent. These processing solutions, however, suffer from greater variations of pH and composition.
JP-B 44733/1992 and 44734/1992, primarily relating to the amine amount, describe a developer containing more than 0.07 mol/liter (7.7 gram/liter) of dihydroxybenzene. However, since this developer is for block copy printing material and in practice, contains dihydroxybenzene in an amount of up to 12 gram/liter, it is inadequate as a processing solution for the lithographic printing material which requires improvements in stability, plate wear and ink receptivity.
Lithographic printing plates which use an internal latent image type direct reversal emulsion for producing negative diffusion transfer silver images have the problems that no satisfactory reversal images are obtained and plate wear and ink receptivity are aggravated if the processing solution has higher pH or contains less hydroquinone. There is a desire to improve the printing plate in these respects.